I was considering buying this lens, but after reading your review i'm not sure it's still a good idea.
despite the fact I just sold the previous tele I had, a tamron 70-300 4.5-5.6 (bought 180€).
I was not really happy with that one, too slow AF and bad quality image.
I finally managed to use it the way I wanted, from f8 to f11 and at 1/400s minimum.
Taking a picture with one of those two settings below and it was a mess.
So i'm still wondering, even if the IQ is not clearly here, is it worth the price when you come from a crapy bottom-class tamron ?

I was playing with a 350D, from now on I have bought a used 20D because of the (at least) 2 month delay to receive the 5D² I ordered in june (and still not received)

Hello esquimo_2ooo, and welcome to the friendly Camera Labs forum!
To enjoy your stay here please have a look at the house-rules!
I've personally not found the "perfect" tele-zoom for myself, but when you own a Canon-body, the Canon 100-400 IS should be the best in this class. Read a review that I trust here. But it's pricey!
So aiming a little lower on the price scale the Sigma 120-400 OS is certainly not bad and it's in a different class from the Tamron 70-300 4.5-5.6. Just don't expect too much from the image stabilization: It looks good through the viewfinder but does not have the same effectivity than Canon IS or Nikon VR.
If you bother about sharpness read this post in my review. You can see that the earlier problems with sharpness mostly came from shake.

I got this lens just recently having airshows (and motorsports) in mind. After my first action tests few days ago i have the following questions and observations. What i wanted to investigate with testing was what OS mode really is suitable for shooting flying aircraft in airshows at a distance and which mode is best for panning with low shutter speeds (or if OS off is better overall) and if there are issues with speeds faster than 1/500 and OS ( as there are stories supported even by Thom Hogan about VR causing blur issues @ speeds faster than 1/500 . http://www.bythom.com/nikon-vr.htm )

I couldn't detect obvious differences between OS off, OS1, OS2, and over or lower than 1/500 speeds. Actually i think my sharpest shots were strangely with slow panning speeds like 1/125 @ apertures over 11 (14-16) with either OS off or 1 or 2 mode. But i see in most of pics, in any OS configuration, (except maybe the ones that seem definitely accurately focused and with 11+ apertures) there is something like aberration: sharp edges of white areas blur or shift a little to the right and up while sharp edges of black areas blur or shift a little to the left and down. Without this all pics would had been much sharper and cleaner. What is this? Is it a case of CA? Neither Capture NX or Photoshop were able to affect this with their CA tools. Or is it just not accurate focus ? eg : DSC_0305.jpg, DSC_0174.jpg from the link below :

The only conclusion i could possibly come to so far is that i should probably rather shoot with as much as higher F number as possible (to increase depth of field?) I still have no conclusion as to which OS mode is better for shooting an aircraft performing aerobatics (probably with 1/500+ speed) and which mode is better for slow shutter speed panning (to motion blur the surroundings) , always everything handheld ofcourse. Or if both OS modes affect AF speed significantly so maybe the AF speed gained with OS off produces sharper pictures than OS on where AF deceleration would cause maybe AF to miss as the D90 in AF-C mode takes the picture even if its not focused.

Any ideas, suggestions? Thanks

PS something else i am concerned about is that holding the lens while shooting, from the zoom ring (or even zooming in and out during fast shooting action) i think my hand also is in contact in many cases with the focus ring as well and i wonder if this contact messes the focus (especially when i am zooming as my left hand supporting at the same time the heavy lens would automatically get firmer on the lens in order to get more grip). Zoom and focus rings are placed alarmingly close i think for an HSM lens. The safest bet would be maybe to avoid as much as possible to hold from the zoom ring and hold the lens form the tripod colar grip instead ? ...

Hello monographix, and welcome to the friendly Camera Labs forum!To enjoy your stay here please take the time to read the house-rules!----I've had a look at your images and I'd like to make a few comments:- This was a fast moving sport you were shooting! This was a very challenging situation for your gear and I think that the images are pretty good for that!- with moving subjects plus tracking there is alway a pretty high probability that shake, blur and focus-tracking errors reduce image quality somewhat.- to see "pure" optical performance under these circumstances is pretty hard.- but yes: the 120-400 has something called residual spheric aberrations that work the way you described: depending on the position relativ to the optical axis (and the focus) you get white smearing/haloing into black. The image in my post here is testament to that.- and finally: I think that with shutter speeds of 1/500 sec I'd go for OS=OFF (with any lens), just to make sure that the image stabilization does not ad some unwanted IQ-degradation.

Hi Thomas , nice to meet you and greets to everyone here and thanks for the input!

So it is true indeed that any stabilization system may possibly cause IQ degradation on speeds faster than 1/500? It would had been interesting if there was some thorough research somewhere about this as i see it has been discussed a lot for years now and there are no definite conclusions (eg most people i met in airshow photography forums that use this lens prefer to shoot with OS1 on, more pro action / sports photographers is claimed they turn it off as you -and the other Thom- suggest)

- What are the chances that those aberrations may be reduced at lets say 13-18 apertures ? Did you try this viewtopic.php?p=54079#p54079 with larger f?

- I am trying to form a checklist of procedures and dos / dont's to get the best of this lens when i will be using it on rare for me to attend -but very excited about- airshows (thus increasing the importance of keepers rate)

- It was interesting for me that in this row of test action shots (arund 350 i think) most of my few sharpest ones were taken at 1/125 (and 14-18 aperture)

Quote:

with moving subjects plus tracking there is alway a pretty high probability that shake, blur and focus-tracking errors reduce image quality somewhat.

Well, theoretically in-lens image stabilization might degrade image quality like some sort of decentering defect: It moves lens-groups around in the construction that optimally should sit untilted and perfectly aligned in the center of the optical axis for optimum quality. BUT:- I have tried to get one shot that made a visible difference between IS/OS/VC/VR=ON and IS=OFF. I couldn't manage to find proof.- If this effect exists it will show itself at fast shutter-speeds (like say 1/500 sec) and slow(er) shutter speeds. Because shooting through decentered glass is looking the same regardless of how fast you shoot.

What I'd do is look more on the up-side of IS/OS/VC/VR: under what conditions is it likely you can see a positive effect from stabilization?Well, I can only offer my personal experience and thoughts on that, don't expect this to be conclusive:- You profit more from IS at lower speeds: Every control-loop needs some time to react and adjust to do any good: So at 1/125 sec you should see more of an effect from IS than at 1/500 sec. At 1/30 of a sec even more so - but then you might be at such a low speed that the shot is ruined even with IS=ON. - If you refrain from pixel-peeping (i.e. don't look at 100% magnification) the effect from IS may look better than at 100% viewing. My experience says that IS is much better at stabilizing to an accuracy of 2x2 pixels than at 1x1 pixel. This chimes well in with my warning on the type of fast-moving targets that you're shooting. On a pixel-level I'm pretty sure that you'd almost always might find some fault.

Just did a focus check on my 120-400 sample. First time ever i am doing this. Uploading the results in dropbox but it takes time, they are large files i guess.

- There is slight back focus. ("3cm_separation" & "6cm_separation" folders.) Increasing the depth separation between batteries to 6 cm clearly separated focus between all three of them .

- Live view AF focuses much more accurately but still (maybe) with some slight back focus. ("Live_View" folders) [note : Live View AF needs a little assistance to focus (i have to rotate the ring manually at aproximately the correct distance first) Low light conditions here now. Viewfinder AF though has no trouble to focus immediately on target.]

I recently had a chance to try this lens on a Canon T4I, hand held on a overcast/hazy day. Considering everything, I think it did a respectable job

I got shots handheld I couldn't have with my A-1 and 400mm lens. The place was crowded and hard to get a clear shot of almost anything, but I got what I could. I bought what I could afford and I got shots that I'm happy with. So far this lens works for me.

The lens has been back from Sigma service and since then it works great. Its sharp, its accurate, its lightweight, its compact, its fast. If a sample works as its supposed to, i believe its the best on a budget alternative to Canon 100-400 / Nikon AF-S 80-400, offering almost equal performance.